1. Field of Invention
The present invention relates generally to a method for the growth of germanium crystals and, more particularly, to a modified Czochralski method using a quartz shield for growing high purity germanium (HP-Ge) crystals in load cell controlled stainless steel furnace. More particularly the present invention relates to the automatic diameter control of high purity germanium crystals between 3 cm and up to at least 15 cm in diameter.
2. Description of the Prior Art
Germanium-based experiments have recently reported a possible dark matter signature (CoGeNT) [1] and have claimed discovery of neutrinoless double beta decay [2, 3] making germanium a preferred target for use in future experiments. Five research groups, Super-CDMS, CRESST, GERDA, MAJORANA, CDEX [4-9], are working to detect dark matter utilizing large-scale HP-Ge crystal detectors with ultra-low internal radioactive backgrounds. The sensitivity of the germanium-based experiment is related to the size of HP-Ge crystals. Large diameter (>3 inch) HP-Ge crystals have higher sensitivity and supply a bigger volume for collecting the signal, especially from weakly interacting massive particles (WIMP).
For HP-Ge crystals, the impurity level should be lower than 2×1010 cm−3 and the local density of dislocation is in the range of 102-104/cm2. Currently, the HP-Ge single crystal used for the largest HP-Ge detectors weighs 4.8 kg and is ∅10×12 cm2 [10]. It is tremendously difficult to grow such big HP-Ge-detector grade crystals. In the 1970s, HP-Ge crystal growth with diameters in the range of 2˜3.5 cm was discussed [11, 12]. However, from then on, there has been less work focused on the research of crystal growth, especially for large size (>6 cm in diameter) crystals.
High purity germanium crystals (13N) were grown by the Czochrolski method in a hydrogen atmosphere. Usually, it is grown in a quartz tube crystal grower (W. L. Hansen, Nuclear Instruments and Methods, 94(1971)377-380). A graphite crucible was located at the outside of quartz crucible to absorb the radio frequency wave to generate heat to melt the germanium ingot. Ceramic insulation was used to keep the thermal field stable. The insulation and graphite crucible must be very pure, otherwise, when the temperature of graphite crucible goes to the melt point of germanium, some phosphorus, arsenic or boron oxides in the insulation materials and graphite crucible will react with hydrogen and introduce n- or p-type contamination into the germanium melt and make the purity of the crystal decrease. The highly pure insulation and graphite crucible represent huge cost. Another deficiency is that this kind of design is difficult to operate during loading of raw materials into the quartz crucible and in taking the crystal out. Many companies still use this kind of design to grow HP-Ge crystals. Large size detector grade crystal needs large quartz tube furnaces. This is the reason why the largest size HP-Ge crystals are only 4 inches (100 mm) in diameter. This is one of the reasons why the price of HP-Ge crystals is about ten times of germanium ingot.
Stainless steel furnaces are used to grow electric grade and infrared grade germanium crystals. The fabrication technology of stainless steel furnaces for the Czochralski (CZ) method crystal growth is very mature and the automatic control of it is very accurate.
Stainless steel furnaces can get a very high vacuum (>10−6 Torr), which is very important for decreasing the SiO2 precipitation in HP-Ge crystals. The stainless steel furnace is very safe to operate with H2 gas and can grow large size germanium crystals. They are currently able to grow infrared grade germanium crystals up to a size of 300 mm. Generally, people in the art believe that the stainless steel chamber will introduce contaminations, which is likely the reason no company currently uses a stainless steel furnace to grow HP-Ge crystals.
A further obstacle is the degree of training and skill of the human operators of the traditional CZ methods required to produce large, high quality crystals. Indeed, one limitation in this technology is the limited number of people with adequate training and skill. A method is needed, which will allow operators of relatively low skill to produce large, high quality crystals.